'type' declarations

[Simon Peyton-Jones]

You're quite right.  GHC has a simple but non-performant representation
of type synonyms in types, so as to be able to generate good error
messages,  In particular, the type

	S t

where S is a type synonym defined by 'type S a = s', is represented as

	SynNote (S t) (s [t/a])

That is, (S t) is represented by *both* its un-expanded and expanded
form.  

The SynNote is ignored by unification, but the un-expanded form is
useful for error messages.  Unfortunately, t is duplicated, as you can
see, and that leads to the behaviour you describe.

I don't see myself fixing this soon, at least partly because I can't see
an obvious way to fix it that doesn't lose error message behaviour.

I'm going to open a SourceForge bug for it.  If anyone has good ideas,
let me know.

Simon

| -----Original Message-----
| From: glasgow-haskell-bugs-bounces at haskell.org
[mailto:glasgow-haskell-bugs-
| bounces at haskell.org] On Behalf Of Iavor Diatchki
| Sent: 17 February 2005 01:27
| To: glasgow-haskell-bugs at haskell.org
| Subject: 'type' declarations
| 
| hello,
| ghc seems to be having trouble with 'type' declarations.
| while compiling (i guess kind checking is the correct word here)
| the following program for a very long time, ghc (6.2) runs out of
300Mb of heap.
| 
| module Test where
| 
| type S          = Maybe
| type S2 n       = S  (S  n)
| type S4 n       = S2 (S2 n)
| type S8 n       = S4 (S4 n)
| type S16 n      = S8 (S8 n)
| type S32 n      = S16 (S16 n)
| 
| type N64 n      = S32 (S32 n)
| 
| type N64'       =
|   S ( S ( S ( S ( S ( S ( S ( S (
|   S ( S ( S ( S ( S ( S ( S ( S (
|   S ( S ( S ( S ( S ( S ( S ( S (
|   S ( S ( S ( S ( S ( S ( S ( S (
|   S ( S ( S ( S ( S ( S ( S ( S (
|   S ( S ( S ( S ( S ( S ( S ( S (
|   S ( S ( S ( S ( S ( S ( S ( S (
|   S ( S ( S ( S ( S ( S ( S ( S (
|   Int
|   ))))))))
|   ))))))))
|   ))))))))
|   ))))))))
|   ))))))))
|   ))))))))
|   ))))))))
|   ))))))))
| 
| if i remove the N64 definition things work.  i guess something
| exponential is happening
| (substitution?).
| 
| -iavor
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